A motor vehicle generally comprises an electric starter for starting the internal combustion engine of the vehicle. The starter converts electric energy stored in a vehicle battery into mechanical energy for driving the engine to rotate, thereby the engine is started.
The starter mainly comprises a direct-current type electric motor, a transmission mechanism, a control device and the like. When the vehicle engine is to be started, the electric motor is energized by a direct current from the battery to generate a rotational torque. The rotational torque is transmitted to a driven gear on a flywheel of the engine via the transmission mechanism to drive a crank shaft of the engine to rotate. The transmission mechanism comprises a speed reduction mechanism coupled with an output shaft of the electric motor, an overrunning clutch coupled with the speed reduction mechanism, a driving shaft coupled at its back end with the overrunning clutch via a spline, and a pinion mounted to a front end of the driving shaft for driving the driven gear. The driving shaft is axially slidable relative to the overrunning clutch by means of the spline. The control device is configured to control the operation of the electric motor and the axial movement of the driving shaft, so that the pinion is engaged with or disengaged from the driven gear.
When a driver starts the vehicle by an ignition key, the starter is switched on, the pinion is moved forwards with the driving shaft, so that the pinion is engaged with the driven gear, and then the engine is started. Once the rotational speed of the engine is increased to a certain value, the driving shaft moves backwards together with the pinion, so that the pinion is disengaged from the driven gear.
The rotational speed of the internal combustion engine has instability due to structure and operation principle of the engine. Specifically, in the four strokes of the engine, only the expansion stroke contributes to acceleration of the rotation of the internal combustion engine, while the other three strokes all create resistance, not propulsion, to the rotation of the engine. For a four-stroke internal combustion engine, the four strokes are completed by two turns of rotation. During the operation of the starter, the starter generally drives the internal combustion engine until it completes 3 to 4 turns of rotation. In this stage, the starter is subjected to fluctuation in load, and sometimes the load is even a minus load. As a result, the starter is forced to accelerate and decelerate in the procedure of driving the internal combustion engine. In this procedure, the load on the starter may be increased to be more than two times the maximum output torque of the starter. Such a load is the maximum load in the operation of the starter. This maximum load is generally an input parameter in the design of the starter, and is a key factor in determining capacities of components of the starter. There is also another case in which a malfunction may occur in the internal combustion engine or the system of it during the driving procedure of the starter, which may results in abrupt stop or even reverse rotation of the starter. In this case, the internal combustion engine applies an extremely high, or even destructive, impact load to the starter.
Under the same condition, lowering down dynamic load may result in reduced design size of the starter, less strength requirement of parts, and lower product cost. On the other hand, under the condition of the same strength, the service life of the starter can be increased.
In the vehicle starters of prior art, axially engaged friction disks are commonly used for protecting the electric motors. Defects found in such protection means comprise limited torque transmission ability of axially engaged friction disks and relatively complex structure of the protection means itself